The present invention relates to a fluid pump applied to an air pump constituted by a displacement blower used as a supercharger for an internal combustion engine and, more particularly, to the structure of a rotor.
The two-lobe rotors of displacement blowers constituting air pumps are roughly classified into cycloid type rotors constituted by epicycloid and hypocycloid curves, and envelope type rotors which are designed such that the inner transitional surface of one rotor serves as the envelope curve of the outer transitional surface of the other rotor.
Such a two-lobe blower is mounted, as a supercharger for an internal combustion engine, in a vehicle, as shown in, e.g., FIGS. 5 and 6. FIGS. 5 and 6 show states in which conventional pumps are mounted in internal combustion engines. A blower (S/C) a shown in FIG. 5 is rotated/driven by the operation of an electromagnetic clutch i coupled to an engine (Eng) b through a belt c. The blower a is designed to pressurize air supplied through an air cleaner e, an air flowmeter f, and an inlet pipe g, and distribute the pressurized air to each combustion chamber of the engine b through a slot valve d and a surge tank h. A blower (S/C) a shown in FIG. 6 is interposed between an engine b and a slot valve d and is designed to equally distribute pressurized air to each combustion chamber of the engine b.
As such a displacement blower, the blower disclosed in U.S. Pat. No. 5,039,289 (to be referred to as a Wankel blower hereinafter) is a representative blower. The rotor shape of this Wankel blower will be described below with reference to FIGS. 7 and 8. FIG. 7 shows only the first quadrant of a coordinate system defined by the center line of a large arcuated surface (to be described later) as the X axis and the center line of a small arcuated surface (to be described later) as the Y axis. FIG. 8 shows a meshed state of rotors. FIG. 8 shows only part of the rotors.
A rotor 1 of a Wankel blower has a large arcuated surface 2 having a radius R and inclining at an angle of about 25.degree. with respect the X axis, and a small arcuated surface 3 having a radius r and inclining at an angle of about 45.degree. with respect to the Y axis. In contrast to a general Roots blower, the Wankel blower has a good sealing property. A side surface connecting the large arcuated surface 2 to the small arcuated surface 3 includes an inner transitional surface 4 constituted by a flat surface overlapping a straight line 1.sub.1 inclining at an angle .alpha. (45.degree.) with respect to the X axis and passing through a center O, and an outer transitional surface 5 constituted by a flat surface overlapping a straight line 1.sub.2 crossing the straight line 1.sub.1 at an angle .beta. (120.degree.). The small arcuated surface 3 and the inner transitional surface 4 are connected to each other through an arcuated surface 6 constituted by a concave surface. The inner transitional surface 4 and the outer transitional surface 5 are connected to each other through a convex surface 7 constituted by, e.g., an epicycloid curve or a circular curve. Note that a neck denoted by reference numeral 8 in FIGS. 7 and 8, at which the outer transitional surface 5 and the large arcuated surface 2 cross each other, is not specifically chamfered.
Each rotor 1, of the Wankel blower, which has the above-described shape is designed as follows. Provided that a distance (inter-axis distance) O.sub.1 O.sub.2 between central points O.sub.1 and O.sub.2 of the rotor 1 is 2L the relationship between the radius r of the small arcuated surface 3 and the radius R of the large arcuated surface 2 is given by r=2L-R, and the radii r and R are determined by the distance between rotation axes (to be described later). In addition, the inner transitional surface 4 and the outer transitional surface 5 have a boundary line of an angle of 45.degree., and the inner transitional surface can be formed by a cycloid curve or an envelope curve.
The inner and outer transitional surfaces 4 and 5 of each rotor 1 of the Wankel blower are constituted by flat surfaces so that the surfaces 4 and 5 of the respective rotors 1 are meshed with each other at an angle, unlike a pair of rotors of a general Roots blower which are meshed with each other to smoothly roll. With this arrangement, at the instant that the two transitional surfaces 4 and 5 are meshed with each other, a triangular closed space (to be referred to as a dead space hereinafter, although it is described as a gas pocket in the official gazette) S is formed between the two rotors. Since the dead space S is not quickly released and narrowed upon rotation of the rotors 1, a problem tends to occur in the Wankel blower as pressurized air in the dead space S is pressurized, and a drive resistance for driving the blower can be reduced.
Although the Wankel blower is advantageous in reducing the drive resistance, various problems are posed in practical applications.
More specifically, the dead space S causing a loss volume is large, and the loss volume reaches about 5% of the inlet air amount, resulting in a low volumetric efficiency. In addition, an increase in the temperature of the blower itself is very large, and the blower produces large noise.
In consideration of these drawbacks of the Wankel blower, displacement blowers having improved rotors are disclosed in Japanese Patent Publication No. 3-38434 (to be referred to as the former hereinafter) and Japanese Patent Laid-Open No. 63-248992 (to be referred to as the latter hereinafter). The rotors of these blowers will be described below with reference to FIGS. 9 and 10.
FIG. 9 shows part of a rotor of a conventional air pump disclosed in Japanese Patent Publication No. 3-38434. FIG. 10 shows part of a rotor of a conventional air pump disclosed in Japanese Patent Laid-Open No. 63-248992. As shown in FIG. 9, a rotor 9 of the former has an outer transitional surface 12, constituted by an epicycloid curve, and an inner transitional surface 13, constituted by a hypocycloid curve, which are formed between a small arcuated surface 10 and a large arcuated surface 11 to connect them to each other.
As shown in FIG. 10, a rotor 14 of the latter has an outer transitional surface 17, constituted by an arcuated surface, and an inner transitional surface 18, constituted by an envelope curve, which are formed between a small arcuated surface 15 and a large arcuated surface 16 to connect them to each other such that the central points of the surfaces 17 and 18 coincide with each other on the X axis.
With the above-described rotor shapes, the loss volumes of these two air pumps are greatly reduced to improve the volumetric efficiencies. In addition, an increase in temperature and the generation of noise can be suppressed.
The following problems, however, are expected when the two blowers described above are examined in detail.
Assume that the blower of the former is in a meshed state such as the one shown in FIG. 8. In this case, the inner transitional surface 13 constituted by the hypocycloid curve opposes the outer transitional surface 12 constituted by the epicycloid curve with a design clearance, while the slidable contact surface of the small arcuated surface 10 of one rotor 9 opposes the slidable contact surface of the large arcuate surface 11 of the other rotor 9 with a design clearance.
Immediately before such a meshed state is attained, the outer transitional surface 12, of one rotor 9, constituted by the epicycloid curve, and the inner transitional surface 13, of the other rotor 9, constituted by the hypocycloid curve, are meshed with each other to smoothly roll with the design clearance. For this reason, when the pair of rotors roll while they are meshed with each other, the volume of a wedge-like space formed between the two rotors changes. That is, similar to a general Roots blower, as the wedge-like space formed between the rotors is narrowed, pressurized air in the space is further pressurized to increase the drive resistance. Note that a similar problem may be posed in the blower of the latter.
If, as in the above-described two blowers, rotors having transitional surfaces constituted by small and large arcuated surfaces, cycloid curves, and envelope curves are designed to be sequentially meshed with each other to roll, the clearance formed between a pair of rotors while the rotors roll varies, so that the rotors cannot always be caused to roll in slidable contact with a constant clearance.